Can you backspin other kites? I don't think there is anything unusual about the bs on the SF. Multilazies require more precise timing and slack-giving - if you want to do more than three or four - but backspins are very forgiving.
Clearly this isn't your experience though
When you say lat roll, do you mean half a backspin and it comes back into upwards flight? If so, it's probably a slack problem.
Input wing a little dipped / further away before first rotation?
Full slack with non-input hand? (It shouldn't touch the kite at all - you can drop the non-input strap and backspin indefinitely.)
Fairly soft, short input?
Stroll/lope/run forward, wind dependent?
For a 2.1m kite, the SF demands quite a lot of slack. Try more slack / be more gentle.